An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.
There are some important differences between an applet and a standalone Java application, including the following:
An applet is a Java class that extends the java.applet.Applet class.
A main() method is not invoked on an applet, and an applet class will not define main().
Applets are designed to be embedded within an HTML page.
When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine.
A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment.
The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime.
Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed.
Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file.